Ceramic glaze



United States Patent 3,498,804 CERAMIC GLAZE Clarence M. Head, Jr.,Decatur, Ga., assignor to Lockheed Aircraft Corporation, Burbank, Calif.No Drawing. Filed Mar. 25, 1963, Ser. No. 267,829 Int. Cl. C03c 5/00,5/02 US. Cl. 10649 1 Claim This invention relates to a ceramic glaze forrefractory castables having a low coefficient of thermal expansion.

In recent years refractory castables, such as the fused silicarefractory cements, have been developed which have an extremely lowcoefficient of thermal expansion (in the range of 0.5 to 7.O 10 F.). Byreason of their low coefficient of thermal expansion, such refractorycastables are adapted for use in a variety of applications and areparticularly suited for use in heated molding dies for the molding ofparts to close tolerance without concern for expansion or contraction ofthe die. Such refractory castables have the important limitations,however, of being quite porous and rough, thus preventing the molding ofsmooth surface parts. To alleviate these limitations, much research hasbeen done in recent years toward the development of a smooth glazecoating having a coefficient of thermal expansion compatible with orcomparable to that of the refractory castables having a low coefficientof thermal expansion in the range of O.5 10- to 7.O 1O'- F.

This invention eleminates the disadvantages and limitations of suchprior art refractory castables having a coefiicient of thermal expansionby providing a smooth, abrasion resistant, and thermal shock resistantcoating therefor, having a coefficient of thermal expansion compatiblewith that of refractory castables whereby transient temperature changeswill not cause failure of the glaze coating. Moreover, the glaze maturesat the same temperature as fired refractory castables, and therefore maybe fired along with the refractory castable, thus eliminating theadditional expense of two tiring operations. Further, since the fusedsilica refractory cements revert to cristobalite above 2,400 F. and thestrength of calcium aluminate refractory castables decreases excessivelyabove that temperature, the maturing temperature of the glaze ismaintained below 2,400 E, in the range of 1,800 P. to 2,400 F. The glazealso having good abrasion characteristics results in it having a longlife.

Accordingly, it is an object of this invention to provide a ceramicglaze coating for refractory castables having an extremely lowcoefficient of thermal expansion.

Another object of this invention is to provide an abrasion resistant andthermal shock resistant ceramic coating having an extremely lowcoefiicient of thermal expansion.

Still another object of this invention is to provide a ceramic glazecoating having an extremely low coelficient of thermal expansion whichhas a maturing temperature in the range of 1,800 F. to 2,400 F.

Other objects and advantages will become apparent from the followingdescription.

Generally stated, this invention comprises a smooth, abrasion resistant,thermal shock resistant ceramic glaze coating having a low coefficientof thermal expansion particularly adapted for the coating of refractorycastables having a low coefficient of thermal expansion.

Toward this end there is provided a ceramic glaze which, when compoundedwithin the ranges listed in the below glaze formula and by following theprocedures ice detailed hereinafter, yields a ceramic glaze meetingthese criteria. The general properties of the glaze ingredients arelisted in the glaze formula for convenience.

GLAZE FO RMULA Range (pbw.) Properties of ingredient Glass former andflux.

Flux and brilliance.

Abrasion resistance, refractory qualities acid resistance, reducecrawling tendencies.

Glass former and flux, increases fluidity of glass.

Abrasion resistance, refractory qualities.

Glass former, abrasion resistance, refractory qualities.

Reduces crazing, improves alkali resistance.

In order to more fully illustrate the instant invention, the followingexamples are presented.

EXAMPLE 1 A standard frit composition was prepared by melting togetherfor about one hour at about 2,400 F., a mixture of the followingingredients:

Parts by weight Litharge 18.0 Potash spar 20.1 Boric acid 13.2 Silica18.6

Lithium carbonate 1.0 Chrome oxide 0.4 Zircon 5.6

The calculated composition of the resulting frit, expressed in weightpercentages, was as follows:

quenched in water. The quenched frit was then ground to minus 200 mesh,and a mill addition of potash feldspar (10.1 parts by weight), kaolin(7.4 parts by weight), spodumene (14.9 parts by weight) and Cr O (1.0part by weight) was made. A glaze slip was then made by adding 60percent water by weight. The glaze was then applied to the commerciallyavailable fused silica refractory cement, known'by the trade name ofGlasrock, and fired at approximately 2,000 F. on an 8-hour schedule. Theresultant glaze was smooth, glossy and of a deep green color due to thepresence of the Cr O Using the same procedure as described in Example 1,other frit compositions and glazes were prepared by mixing variousquantities of ingredients to produce frits having a variety ofcompositions; and ceramic glazes were prepared from such fritcompositions falling within the ranges of this invention. The amounts ofingredients employed, the glaze compositions along with the smelting andfiring temperature, and the observed qualities of the glazles arerecorded below in Table I along with the data of Example 1.

Ingredients added 1 2 3 4 Litharge 18. 19. 4 18. 3 17. 3

Potash spar- 30. 2 17. 6 16. 6 15. 7

Boric acid 13. 2 14. 0 13. 2 12. 5

Silica 18. 6 20. 0 18. 8 17. 9

Chrome oxide. 1. 4 0. 4 0. 4 0. 4

Lithium carbonat 1. 0 0. 8 0. 8 0. 8

Kaolin 7. 4 8. 0 7. 6 7. 2

Spodumen 14. 9 20. 0 20.0 17. 8

Zircon 5. 6 4. 7 8. 9

Weight percent (examples) Glaze composition 1 2 3 4 smelting tern 2, 4002, 400 2, 400 2, 400

Firing temp. F.) 2, 000 2,000 2, 200 2, 400

In each of the examples, the resulting glazes were smooth, glossy,abrasion resistant, thermal shock resistant and had good continuity.Although each of these glazes had good continuity, those of Examples 1,3 and 4 were the most continuous and of these, the glaze of Example 4was more continuous than the glaze of Example 3 due to the beneficialeffects of their high ZrO content.

Still another example of a glaze according to the present inventionconsists essentially of PbO in the amount ofv about 19%; alkaline metaloxide in the amount of about 5% and selected from the group consistingof K 0, M 0 and mixtures thereof; A1 0 in the amount of about 12%; B 0in the amount of about 9%; SiO in the amount of about 49%; and Zr0 inthe amount of about 6%; all percentages expressed on a weight basis.

An important characteristic of the ceramic glazes of this invention istheir good abrasion resistance. The A1 0 Cr O and ZrO give the glaze itsabrasion resistance characteristics; and to assure that optimum abrasionresistance would be obtained, the ceramic glaze of Example 2 was testedto obtain an abrasion coeflicient. Then the ingredients ZrO and Cr Owere added to the Example 2 glaze in incremental amounts in parts byweight to determine their efiect on the abrasion resistance of the testglaze. The glazes thus formed fall within the range contemplated-by thisinvention; however, for convenience the weight percentages of theircomponents have not been listed in Table 1. The abrasion tests wereconducted on a power hack saw with silicon carbide 180 grid sand papermounted on the saw blade as the abrasive media. A relative abrasionresistance coefficient for each sample was determined by the formula:

R Time (sec.) to make 1%" long cut through glaze sample Average glazethickness (inches) X 1000 The results were as follows:

(1) Example 2 alone (R=16)' (2) Example 2 glaze+5% Zr0 (R=20) (3)Example 2 glaze+5% ZrO +1% Cr O (R=20) (4) Example 2 glaze+5% ZrO +2% CrO (R=2l) (5) Example 2 glaze+10% ZrO +2% Cr O (R=26) The resultsindicate tha the Example 2 glaze has good abrasion resistancecharacteristics, that the addition of Cr O thereto in small percentagesdoes not materially increase the abrasion resistance of the glaze, andthat the addition of Z-rO in small percentages does materially enhancethe abrasion resistance of the glaze. Accordingly, to obtain optimumabrasion resistance, it is preferred that the glaze of this invention beformulated with between 3% to 10% Zr0 by weight. If a clear glaze isdesired, the Cr O which gives the glaze a deep green color, may beeliminated without adversely affecting the abrasion resistance qualitiesof the glaze. Moreover, since the addition of Cr O in small quantitiesto the glaze produces an excessive increase in the maturing temperatureof the glaze, the quantity of Cr O employed should be maintained quitesmall. The principal advantage obtained by the use of CT O in the glaze,when the glaze is applied to the white and off-white refractory cements,is that it makes possible the detection of faulty application and/orchipping of the glaze. Also, visual and microscopic ex- 1 amination ofthe glazes formed for the abrasion test disclosed that micro-crazing ofthe glaze decreased in direct proportion to the ZrO added. Theseobservations along with the observations of Examples 1, 3 and 4 indicatethat to optimize continuity (minimize crazing), the most desirableamount of ZrO included in the glaze formula to be within the range of 3%to 10% by weight.

Resistance to thermal shock is a very desirable attribute of the ceramicglazes of this invention. To assure that the glazes have good thermalshock resistance, thermal shock tests were conducted on the Example 1glaze. A specimen was quenched from 2,000 F. to the smooth surface of aDry Ice 'block 1 10 P.) where it remained for ten minutes at which timeit could be handled continuously. The specimen was then put into thefurnace at 2,200 F. for ten minutes. This was more than sufiicient timeto raise the glazed surface temperature to 2,200 F but not enough timeto permit significant re-melting of the glaze. The specimen sustained 10cycles before crazing became significant (hairline). Small glazefragments broke from the specimen on the 13th cycle; and the area ofglaze failure had reached 10% of the total area by the 16th cycle. Theseresults compare quite favorably with norms established in the artwherein failure after one to two cycles indicates very poor thermalshock resistance, failure after five to seven cycles indicates very goodthermal shock resistance, and failure after more than seven cyclesindicates excellent thermal shock resistance. From the above descriptionand the examples pro sented, first compositions containing variousconstituents have been produced from which ceramic glazes for refractorycements have been prepared. These glazes are smooth, glossy, thermalshock resistant, abrasion resistant and have an extremely lowcoeflicient of thermal expans1on.

What is claimed is: 1. A ceramic glaze consisting essentially of PhD inamount of about 19 percent; alkali metal oxide selected from the groupconsisting of K 0, U 0 and mixtures thereof, said alkali metal oxidesbeing present in amount of about 5 percent; A1 0 in amount of about 12percent; B 0 in amount of about 9 percent; Si0 in amount of about 49percent; and 210 in amount of about 6 percent; all percentages expressedon a weight basis.

References Cited,

UNITED STATES PATENTS 2,844,693 7/ 1958 Rigterink 10649 2,969,293 1/1961 Smith 10649 3,203,815 8/1965 Michael 10649 3,228,548 1/ 1966 Butler10653 HELEN M. MCCARTHY, Primary Examiner US. Cl. X.R. 10653

1. A CERAMIC GLAZE CONSISTING ESSENTIALLY OF PBO IN AMOUNT OF ABOUT 19PERCENT; ALKALI METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF K2O,LI2O AND MIXTURES THEREOF, SAID ALKALI METAL OXIDES BEING PRESENT INAMOUNT OF ABOUT 5 PERCENT; AL2O3 IN AMOUNT OF ABOUT 12 PERCENT; B2O3 INAMOUNT OF ABOUT 9 PERCENT; SIO2 IN AMOUNT OF ABOUT 49 PERCENT; AND ZRO2IN AMOUNT OF ABOUT 6 PERCENT; ALL PERCENTAGES EXPRESSED ON A WEIGHTBASIS.